Electrophotographic marking is a well known and commonly used method of copying or printing original documents. Electrophotographic marking is typically performed by exposing a light image representation of an original document onto a substantially uniformly charged photoreceptor. In response to that light image the photoreceptor discharges so as to create an electrostatic latent image of the original document on the photoreceptor's surface. Toner particles are then deposited onto the latent image so as to form a toner powder image. That toner powder image is then transferred from the photoreceptor, either directly or after an intermediate transfer step, onto a substrate such as a sheet of paper. The transferred toner powder image is then fused to the substrate using heat and/or pressure. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the creation of another image.
The foregoing generally describes a typical black and white electrophotographic printing machine. Electrophotographic printing can also produce color images by repeating the above process for each color of toner that is used to make the color image. For example, the charged photoconductive surface may be exposed to a light image which represents a first color, say black. The resultant electrostatic latent image can then be developed with black toner particles to produce a black toner image which is subsequently transferred and fused onto a substrate. The process can then be repeated for a second color, say yellow, then for a third color, say magenta, and finally for a fourth color, say cyan. If the toner particles are placed in a superimposed registration the desired composite color image is formed on the substrate. This process is sometimes referred to either as the REaD process (Recharge, Expose, and Develop) or as the IOI process (Image On Image).
While electrophotographic printing has been very successful, the rapid growth of the computer industry has created a tremendous demand for desktop printing machines, particularly color desktop printing machines. Desirable features of desktop color printing machines include high print quality, high speed printing, low cost, and small size. Those desirable characteristics are difficult to achieve simultaneously. One reason for the difficulty of simultaneously achieving all of the desirable characteristics is that color electrophotographic marking requires numerous processing steps which in the prior art were usually performed using a dedicated device to perform each processing step. The use of dedicated devices increased the cost and size of the electrophotographic printing machines.
Multiple uses of individual devices is known in the prior art. For example U.S. Pat. No. 4,141,648 entitled, "Photoconductor Charging Technique" issued to Gaitten et al., on 27 Feb. 1979 teaches a two cycle electrophotographic copying machine wherein one corona device performs both charging and precleaning functions and wherein another corona device performs both precharging and transferring functions. In the "Background of the Invention" of U.S. Pat. No. 4,141,648 is a discussion of prior attempts to combine charging and transferring in one corona generating device. As discussed, such prior attempts were not entirely successful since the transferring media tended to jam into the grid wires of the corona device and because of nonuniform charge distributions onto the media.
However, color electrophotographic printing involves many more processing steps and is much more sensitive to process variations than electrophotographic black and white printing. Complicating the difficulty of using single devices for multiple uses is the fact that, at least with some color electrophotographic processing techniques, such as image-on-image color processing, charging through developed toner layers and transferring multiple toner layers may be required. The developed toner layers create several problems of interest. First, recharging a photoreceptor to a uniform voltage through an existing toner layer is difficult to do since the presence of toner changes the charge-voltage characteristics of the photoreceptor. Second, toner layers tend to trap charge within their finite thicknesses resulting in an inability to discharge these toned areas to the same electrostatic voltage levels as surrounding non-toned regions. The first problem makes the recharging of a photoreceptor with developed toner layers difficult. The second necessitates the use of special charge neutralizing types of recharging systems and ultimately complicates the transfer of the toner layers onto a substrate and often requires both pretransfer corona and erase treatments. Additionally, REaD Image-on-Image color systems generally utilize Discharge Area Development toner polarity charging whereby the toner is developed in the written image areas and the main charge and toner polarity are equal but are opposite to the transfer polarity. This is as opposed to conventional light lens copying machines which require Charge Area Development and hence equal polarities for the main charge and transfer functions. Because of these problems the method described in U.S. Pat. No. 4,141,648 of making multiple use of charging devices is not compatible with some color printing architectures. Therefore, methods of using individual charging devices for multiple purposes in a color electrophotographic printing machine would be highly desirable.